In wireless communication systems, power amplifiers (PAs) may be used in the final stage of a transmitter to increase power in a transmitted signal. The PAs typically include an active device such as a transistor and convert direct current (DC) power into radio-frequency (RF) power while being driven by a RF input signal. The portion of DC power that is not converted into RF power is dissipated as heat and constitutes a loss of power. The power conversion mechanism that takes place in a PA is described by the power conversion efficiency (PE) and the power added efficiency (PAE).
The power conversion efficiency is typically formulated as the percentage of the DC power that is converted into useful RF output signal power, while the power added efficiency is typically formulated as the percentage of DC power that is converted into useful RF output signal power while taking into account the effect of the RF input (or drive) signal.
It is often the case in wireless communication systems that the PA is the primary consumer of DC power. The power conversion efficiency is therefore an important performance parameter of the PA. A higher power conversion efficiency may for example lead to longer battery lifetime, simpler thermal management requirements and reduced form factor of the PA, amongst others.
It has been shown that the power conversion efficiency can be maximised by ensuring that the DC power is equal to the power ultimately delivered to the load at the fundamental frequency.
This occurs when the sum of power dissipated in the active device and the power delivered to the load at harmonic frequencies is zero. It is known that two necessary and sufficient conditions may fulfil these requirements: firstly by ensuring that the drain voltage and current of the active device do not overlap and secondly by terminating the drain of the active device with appropriate impedances at harmonic frequencies.
These two necessary and sufficient conditions can be fulfilled by ensuring that the drain voltage of the active device is a half sinusoidal waveform and the current waveform is square. PAs typically include a load network which is designed to aid fulfillment of these conditions. An inverse Class-F PA can fulfil the necessary and sufficient conditions to obtain the maximum theoretical power conversion efficiency if the load network presents the following impedances to the drain of the active device:
                              Z          n                =                  {                                                                                                                                        π                        2                                            8                                        ⁢                                                                  V                        DD                                                                    I                        0                                                                              =                                      R                    opt                                                                                                n                  =                  1                                                                                    0                                                              n                  ⁢                                                                          ⁢                  is                  ⁢                                                                          ⁢                  odd                                                                                    ∞                                                              n                  ⁢                                                                          ⁢                  is                  ⁢                                                                          ⁢                  even                                                                                        (        1        )            
where Zn is the impedance presented by the load network to the drain terminal of the active device, VDD is the drain voltage, I0 is the drain current at the fundamental frequency, Ropt is the theoretical optimal resistance and n is the order of harmonic.
However, there are some load network designs which meet the abovementioned necessary and sufficient conditions to some degree. As a result, the efficiency of existing PAs vary somewhat.
Accordingly, there is scope for improvement.